Field of the Invention
The invention relates to a powder-spraying apparatus having simultaneous internal and external charging, which is suitable for a electrostatic powder coating.
Such a powder-spraying apparatus is disclosed in International Patent Application WO 98/24555. The reference teaches a spray gun having a chamber, into which a powder/air mixture can be introduced. The spray gun contains an earth electrode, needle-like internal high-voltage electrodes distributed on a metal ring, and at least one external high-voltage electrode configured as a needle. A high-voltage source configured as a high-voltage cascade supplies high voltage to the electrodes via an electrical connection and the ring.
The at least one external high-voltage electrode is referred to as an additional electrode which can be disposed as desired, with which an electric field and an additional corona can be produced outside, these being intended to increase the deposition efficiency. The electric field generated by the external needle produces a force on the electrically charged particles. In addition, the external corona can effect further charging, and the repulsion between the ions and the charged powder particles can lead to broadening of the spray cloud. Which of the physical influencing factors listed are dominant depends on the quantity of powder expelled, the properties of the powder (good or poor chargeability) and a distance from a work piece.
In the case of small quantities of powder (up to about 150 g/min) and normally chargeable powders, a sufficiently good quantity of charge can be applied to the powder particles using the internal charging on its own. The electric field built up by the external needle has the effect of a force F=E*q acting on the powder particles. Depending on the voltage on the needle tip, the result is a more or less pronounced edge effect or wrap-around. The width of the spray cloud can be influenced by the level of the current.
In the case of greater quantities of powder (greater than 200 g/min), both the internal and the external charging are required for sufficiently high charging of the powder particles.
However, the disadvantage of a configuration with a metallic connection between the electrodes is that the current and voltage on the external needle depend to a great extent on the distance between the gun and the work piece. At a small distance, a much greater current flows, and the voltage is lower than in the case of a large distance. As a result, in particular at high powder expulsion quantities, there may be an increase in the tendency that the deposition efficiency increases with a decreasing distance.
However, this tendency is not desired in practice. Within a technically expedient range (100 mm . . . 250 mm), the aim is that the deposition efficiency should be uniformly high, irrespective of the distance.
In order to produce the high voltage (negative direct voltage), a cascade with a relatively high internal resistance is used. The no-load voltage U.sub.0, that is the output voltage of the cascade in the unloaded state, normally lies in the range of about -80 kV to -100 kV. The internal resistance of the cascade R.sub.i lies in the range of about 400 M.OMEGA. to 600 M.OMEGA.. The actual output voltage of the cascade U.sub.c, together with the output current I.sub.c, is then calculated in accordance with the following equation: EQU U.sub.c =U.sub.0 -I.sub.c R.sub.i
The maximum output current of the cascade is about 80 .mu.A . . . 120 .mu.A. The use of cascades with greater output voltages and smaller internal resistances is not possible, for reasons of safety. The relevant standards in Europe and America require that the air/powder mixture must not be ignited by electric discharges under any circumstances. Therefore, inter alia, the output of the cascade is limited. Further measures for avoiding ignition are bias resistors and the avoidance of electrode configurations that lead to a high electric capacitance.
During powder application, attempts are made to operate at a constant distance. However, for various reasons it is not possible to avoid the distance from the work piece changing (for example at edges).